sound: Remove macro magic from pcm/feeder_eq.c

Turn the FEEDEQ_DECLARE macro into a single inline function
(feed_eq_biquad()). There is no reason to have this as a macro, and it
only complicates the code. An advantage of this patch is that, because
we no longer call the functions created by the macro through function
pointers (biquad_op), we can call feed_eq_biquad() directly in
feed_eq_feed().

Sponsored by: The FreeBSD Foundation
MFC after: 1 week
Differential Revision: https://reviews.freebsd.org/D48032